This invention relates to a tip treatment assembly for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor through a series of slots defined between tip treatment bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
Known tip treatments comprise an annular assembly made up of a plurality of segments in which the slots are formed. These segments are provided with tangs which cooperate with slots in the engine casing to hold the segments in position. The assembly may have more than 100 milled slots, and consequently manufacture is expensive and it is difficult to meet the required tolerances.
Experience has shown that tip treatment segments are likely to exhibit cracking due to vibration as the rotor blades pass the bars between the treatment slots defined between the tip treatment bars and the subsequent interaction between adjacent bars of each segment, since these bars are integral with one another.
Another problem encountered with the prior art is the possible damage caused by a fractured tip treatment bar. Once broken, the remnants of the bar can be displaced from its original location and cause damage.
An object of the present invention is to avoid cracking of tip treatment segments in a gas turbine engine.
A further object of the present invention is to avoid interaction between adjacent tip treatment bars.
A third object of the present invention is to retain fragments of a broken tip treatment bar to prevent further damage.
According to the present invention there is provided a tip treatment assembly comprising a tip treatment bar provided with retaining means. The retaining means of the tip treatment bar cooperates with a retaining element to retain the tip treatment bar with respect to support means of the assembly.
The retaining means may comprise a hole through the bar, through which the retaining element extends. The retaining element may extend through an annular cavity which may be provided by the support.
A common retaining element may cooperate with the retaining means of a plurality of tip treatment bars, and may cooperate with all tip treatment bars in the assembly. Alternatively a separate retaining element may be provided for each tip treatment bar.
The retaining element may be a wire. The retaining means may be provided at one or both end regions of the tip treatment bar.
The tip treatment bars may be formed by stamping from a sheet of material such as a suitable alloy, in which case each bar will have a substantially uniform width corresponding to the thickness of the sheet.
An end region of the bar may be located within a recess provided in the support means, and damping material may be provided within the recess to provide additional damping of vibrations in the bar.